The present invention relates to a means of maintaining compression within the active area of an electrochemical cell, and especially relates to the use of a pressure pad assembly to maintain compression within the active area on the high pressure side of an electrochemical cell.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, in a typical single anode feed water electrolysis cell 101, process water 102 is reacted at oxygen electrode (anode) 103 to form oxygen gas 104, electrons, and hydrogen ions (protons) 105. The reaction is created by the positive terminal of a power source 106 electrically connected to anode 103 and the negative terminal of a power source 106 connected to hydrogen electrode (cathode) 107. The oxygen gas 104 and a portion of the process water 102xe2x80x2 exit cell 101, while protons 105 and water 102xe2x80x3 migrate across proton exchange membrane 108 to cathode 107 where hydrogen gas 109, is formed.
The typical electrochemical cell includes a number of individual cells arranged in a stack with fluid, typically water, forced through the cells at high pressures. The cells within the stack are sequentially arranged including a cathode, a proton exchange membrane, and an anode. The cathode/membrane/anode assemblies (hereinafter xe2x80x9cmembrane assemblyxe2x80x9d) are supported on either side by packs of screen or expanded metal which are in turn surrounded by cell frames and separator plates to form reaction chambers and to seal fluids therein. The screen packs establish flow fields within the reaction chambers to facilitate fluid movement and membrane hydration, and to provide mechanical support for the membrane and a means of transporting electrons to and from electrodes.
In order to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods, pressure pads have traditionally been used within electrochemical cells. Pressure pads have traditionally been fabricated from materials incompatible with systems fluids and/or the cell membrane, such as silicone rubber, thereby requiring that these pressure pads be disposed within a protective encasing.
Pressure pads are typically preloaded to stress levels which counteract those resulting from the pressurization levels of the working fluids of the electrochemical cell plus approximately 50 p.s.i. to guarantee contact between the cell parts. For example, in an electrolyzer which operates at about 400 p.s.i., the pressure pad is designed to handle 650 p.s.i., which constitutes the proof pressure of the unit (1.5 times the working pressure) plus 50 p.s.i. Typically, during operation, these pads are maintained at a compression stress level of from 50 to about 500 p.s.i. Unfortunately, the elastomer materials typically used for the pressure pad take a compression set and chemically break down when compressed to the higher stress levels.
What is needed in the art is an improved pressure pad which maintains uniform compression, can be utilized at pressures exceeding 2,000 p.s.i. and which is compatible with the electrochemical cell environment.
The present invention relates to a unique electrochemical cell comprising: an anode; a cathode; a membrane disposed between said anode and said cathode; an anode screen pack located adjacent to and in intimate contact with said anode; a cathode screen pack located adjacent to and in intimate contact with said cathode; and an electrically conductive pressure pad located adjacent to and in intimate contact with a side of said cathode screen pack opposite said cathode.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.